1. Field of the Invention
The present invention relates to a surface-emitting laser.
2. Description of the Related Art
A vertical cavity surface-emitting laser (hereinafter, referred to as VCSEL) is a laser for emitting a laser beam in a direction perpendicular to an in-plane direction of a semiconductor substrate.
A distributed Bragg reflector (hereinafter, referred to as DBR) is normally used as a reflective layer of the surface-emitting laser.
The DBR is generally formed by alternately laminating a high refractive index layer and a low refractive index layer with an optical film thickness of λ/4.
The surface-emitting laser has such excellent characteristics that a stable single mode is obtained as a longitudinal mode characteristic, a threshold value thereof is lower than a threshold value of an edge-emitting laser, and a two-dimensional array is easily formed.
Therefore, it is expected that the surface-emitting laser will be applied as a light source for optical communication and optical transmission or a light source for electrophotography.
In order to enhance applicability of the VCSEL, a VCSEL which produces a higher output while maintaining a single transverse mode oscillation is desired.
Accordingly, various structures have been considered, and as one of the promising structures, Song et al., Applied Physics Letters Vol. 80, p. 3901 (2002) (hereinafter, referred to as Document 1) proposes a photonic crystal VCSEL in which a two-dimensional photonic crystal structure of a photonic crystal fiber structure is formed in VCSEL.
FIG. 5 illustrates a structure of a surface-emitting laser described in Document 1.
In a surface-emitting laser 600 illustrated in FIG. 5, a lower multilayer reflection mirror 610, a lower spacer layer 620, an active layer 630, an upper spacer layer 640, and an upper multilayer reflection mirror 650 are laminated on a substrate 605.
When voltage is applied to an upper electrode 690 formed on the upper multilayer reflection mirror 650 and to a lower electrode 695 formed under the substrate 605, the active layer 630 emits light, and the emitted light is amplified by a resonator formed of the upper reflection mirror and the lower reflection mirror, whereby a laser oscillation is obtained. As a result, a laser beam is emitted in a direction perpendicular to the substrate.
In a part of the upper multilayer reflection mirror 650, there is formed a current confinement structure 660 including a conductive region 661 and a high resistance region 662.
The current confinement structure is formed through oxidation of an AlGaAs layer or an AlAs layer which has a high Al compositional ratio.
AlxOy which is formed through oxidation of AlGaAs or AlAs has a higher electrical resistance and a lower refractive index compared with AlGaAs or AlAs.
From an upper surface of the upper multilayer reflection mirror 650 to the active layer side, a two-dimensional photonic crystal structure including multiple holes 675 is formed. A defect is provided in a center of the two-dimensional photonic crystal structure.
In a region where the two-dimensional photonic crystal structure is formed, the effective refractive index decreases.
Here, the amount of decrease in effective refractive index described above is less than the amount of decrease in effective refractive index obtained in the region where AlGaAs or AlAs is oxidized.
In optical confinement caused by a refractive index difference, the smaller a refractive index difference is, the larger an area of a wave guide portion where a single transverse mode can be maintained is.
Accordingly, current confinement is conducted by an oxidized aperture and optical confinement in a horizontal direction is conducted by a two-dimensional photonic crystal structure, whereby an emitting area can be increased while maintaining a single transverse mode oscillation, compared with the case where both of the confinement are conducted by the oxidized aperture. In the aforementioned surface-emitting laser of Document 1, the defect size of the two-dimensional photonic crystal structure is made smaller than the current confinement size, with the result that a surface-emitting laser which maintains the single transverse mode and has a larger emitting area can be realized.
However, in the structure where a hole of the two-dimensional photonic crystal is formed from the surface of the upper multilayer reflection mirror, as in the case of Document 1, a deep hole needs to be made for achieving sufficient transverse mode control.
This is because the resonating region having a large light intensity is positioned on the active layer side of the upper multilayer reflection mirror, so the transverse mode control cannot be exhibited sufficiently without the two-dimensional photonic crystal structure.
However, when a deep hole is prepared, the refractive index changes over a long distance in a perpendicular direction within the upper multilayer reflection mirror, which leads to an increase in shift amount of a resonance wavelength of the reflection mirror.
As a result, the reflectance of the upper reflection mirror decreases for a resonance laser beam, which increases reflection loss.
For this reason, a greater emitting area can be secured but resonator performance decreases in the transverse mode control structure, and thus output cannot be sufficiently increased.